Aldebaran Crossing

On the evening of March 4, 2017 (5 Mar 2017 UT), the Moon passed in front of the 0.9-magnitude star Aldebaran (α Tau). Currently, this is the brightest star the Moon ever crosses (excepting the Sun, of course). The favorable first quarter moon (with the nearside 45.9% illuminated) was at a respectable altitude of 31° in the western sky at the time of the dark-limb disappearance. It is not an instantaneous event. The effects of diffraction plus the non-zero angular size of the star ensure that the star disappears over several frames of a high-speed camera, as shown in my video recording of the event below.

At my location, the middle of Aldebaran’s disappearance occurred at 3:56:00.570 ± 0.001s UT. Thin clouds and wind interfered with a pristine recording of the event, but even so you can see in the trace below that the star took about nine frames (~0.017s each) to completely disappear.

The bright limb reappearance was more difficult to time. In my video recording below, you can see that Aldebaran first reappears at about 4:29:39.828 UT. By that time, the Moon had descended to an altitude of 25° and was accompanied by a fair amount of atmospheric turbulence.